Heat exchanger especially adapted for indirect heat transfer by convection



June 3, 1969 N 3,447,602

HEAT EXCHANGER ESPECIALLY ADAPTED FOR INDIRECT HEAT TRANSFER BY CONVECTION Filed June 22, 1967 Sheet l of 7 0 0 o 5/ o o o o g 0 O o o 0 0 0 0 0 I} 0 0 0 0 o 0 0 0 O a 02 0 F 0 0 0 0 0 0 0 0 0 0 0 53 0 0 0 o o 0 0 0 0 0 I o o o 0000oo00000000000o0 0 ooooooi 0000000000000 0000000-000000000 ooooooo OOQOOOQOQOODOOOOOOOOQOR? /9 G J 5 '5 June 3, 1969 N 3,447,602

ALLY ADAPTED HEAT EXCHANGER ESPECI FOR INDIRECT HEAT TRANSFER BY CQNVECTION Sheet Lot? Filed June 22, 1967 Sheet \3 INDIRECT HEAT DALIN ALLY ADAPTED FOR TRANSFER BY CONVECTION HEAT EXCHANGER ESPECI June 3, 1969 Filed June 22, 1967 2 @MW Jar/20 1751222 Q 3,44 7,602 HEAT June 3, 1969 D. DALIN.

HEAT EXCHANGER ESPECIALLY ADAPTED FOR INDIRECT TRANSFER BY CONVECTION Filed June 22, 1967 Sheet of '7 June 3, 1969 DAUN 3,447,602

HEAT EXCHANGER ESPECIALLY ADAPTED FOR INDIRECT HEAT TRANSFER BY CONVECTION Filed June 22, 1967 June 3, 1969 DALIN HEAT EXCHANGER ESPECIALLY ADAPTED FOR INDIRECT TRANSFER BY CONVECTION Filed June 22, 1967 HEAT Sheet 6g of 7' 1 1,, ,I, I A ,7 I l I I 1 z x Jar/20 17512227 3m) June 3, 1969 D. DALIN 3,447 02 HEAT EXCHANGER ESPECIALLY ADAPTED FOR INDIRECT HEAT TRANSFER BY CONVECTION Filed June 22, 1967 Sheet '7 of 7 Jar/251751122 United States Patent Claims ABSTRACT OF THE DISCLOSURE A heat exchanger for the indirect transfer of heat by convection between two flowing media, as for instance water and hot combustion or flue gases traveling along a defined path. The water circulates through a plurality of side-by-side U-shaped ducts mounted in and disposed edgewise to the path of the combustion or flue gases, so that the gases first sweep over one leg of the Ushaped duct units and then the other. The inlet of each duct unit is in the outer extremity of the leg thereof which is last swept by the gases and the outlet is in the extremity of the other leg, so that the donor gaseous medium and the liquid recipient medium flow countercurrent; and because of the cross sectional dimensions of the legs of the U-shaped duct units, the hottest water travels in that portion of each leg which is first swept by the hot flue gases. The U-shaped duct units are either box-like in construction or composed of two groups of parallel tubes connected at one end by a common header which forms the bight portion of the U and connected at their outer ends by separate headers, one for each group of tubes.

This invention relates to heat exchangers for effecting indirect transfer of heat between two flowing media, and refers more particularly to heat exchangers especially intended for use as the convection surfaces of steam boilers and hot water generators.

One of the objections to most steam boilers and hot water generators heretofore available has been the inaccessibility of many of the tubes and ducts which comprise the heating surfaces-both radiant and convection, but particularly the latter. With the exception of the steam boiler and hot water generator of Patent No. 3,200,859, issued to David Dalin, Jan. 24, 1967, the design and construction of steam boilers and hot water generators of the past reflected an inexplicable lack of awareness of the need for having all of the heating surfaces of the boiler or generator accessible for inspection and repair; or, if the need was recognized, an inability to devise a construction in which that need was met.

It is therefore one of the primary purposes of this invention to provide a heat exchanger especially suitable for use as the convection surfaces of a steam boiler or hot water generator, in which accessibility for inspection and ease of removal and replacement are one of its chief attributes.

Another object of this invention is to provide a heat exchanger for effecting indirect heat transfer between two flowing media, which utilizes to the fullest extent the advantages of counter-current flow between the donor and recipient media.

Still another objective of this invention is to provide a heat exchanger of the character described which is exceptionally simple and compact in design and construction, and which takes advantage of modular design to assure low production costs.

With the above and other objects in view which will appear as the description proceeds, this invention resides in the novel construction combination and arrangement of parts, substantially as hereinafter described and more particularly defined by the appended claims, it being understood that such changes in the precise embodiment of 3,447,502 Patented June 3, 1969 the hereindisclosed invention may be made as come within the scope of the claims.

The accompanying drawings illustrate several complete examples of the physical embodiments of the invention, constructed according to the best modes so far devised for the practical application of the principles thereof, and in which:

FIGURES l, 2 and 3 illustrate a heat exchanger constructed in accordance with one embodiment of the invention, FIGURE 1 being a sectional view through the heat exchanger, FIGURE .2 another view thereof on the plane of the line 2-2 in FIGURE 1,, and FIGURE 3 being a perspective view of one of the major elements of the heat exchanger;

FIGURES 4 and 5 illustrate a variation of the heat exchanger shown in FIGURES 1, 2 and 3, FIGURE 4 being a sectional view similar to FIGURE 1, and FIGURE 5 being a sectional view through FIGURE 4 on the plane of the line 5-5;

FIGURES 6, 7 and 8 illustrate another embodiment of the invention, FIGURE 6 being a sectional view through a complete steam boiler or hot water generator equipped with convection surfaces constructed in accordance with this invention, FIGURE '7 being a sectional View through FIGURE 6 on the plane of the line 7-7, and FIGURE 8 being a sectional view through FIGURE 6 on the plane of the line 8-8;

FIGURE 9 is a cross sectional view through one of the tubes of the convection surfaces of the unit shown in FIGURES 6 to 8, inclusive, to illustrate that the tubes are equipped with extended surface;

FIGURES 10, 11, 12 and 13 are plan views of portions of the tubes showing the extended surface elements thereon spaced apart different distances along the tubes for a purpose to be described;

FIGURES 14, 15 and 16 are cross sectional views through individual extended surface elements to illustrate how different elements may be given different thermal conductivity, in accordance with the teachings of the' copending application, Serial No. 639,544, filed May 18, 1967, now Patent No. 3,385,356.

FIGURES l7 and 18 illustrate still another modification of this invention, both "being sectional views through a steam boiler or hot water generator, FIGURE 17 being taken on the plane of the line 17-17 in FIGURE 18, and FIGURE 18 being a sectional view through FIG- URE 17 on the plane of the line 1818; and FIGURES l9 and 20 are longitudinal section views through two slightly different forms of an embodiment of this invention especially adapted for heating oil and such similar uses.

Referring now particularly to the accompanying drawings, and especially to FIGURES 1 through 5, the numeral 5 designates generally a flow passageas, for instance, the flue gas passageway of a steam boiler or hot water genera-tor through which the hot combustion or flue gases which emanate from the primary combustion chamber flow on their way to the stack. This flow passage in the present case is, in effect a box-like structure having a top wall 6, bot-tom wall 7, front and rear walls 8 and 9, respectively, and side walls 10.

Hot combustion or flue gases enter the flow passage through an inlet 11 in the upper portion of its front wall 8 and leave the same through an outlet 12 in the lower portion of the front wall.

Spaced a distance rearwardly of the front wall is a partition 13 which divides the interior of the box-like structure into a main passageway 14 and a bypass 15, both connecting the inlet and outlet. Flow through the bypass 15 is controlled by a damper 15 whichwhen closed as shown in FIGURE 1direc-ts all of the incoming combustion or flue gases into the main passageway 14. Obviously the heating effectiveness of the gaseous medium traveling through the main passageway 1'4 may be controlled by adjustment of the damper; and for the purposes of this invention it is the main passageway 14 which constitutes the flow passage along which the donor medium flows.

Within the main passageway 14 there are one or more side-by side substantially U-shaped duct units 16. These duct unitswhere more than one is usedare all identical in design, size and construction and hence are interchangeable modules. Each duct unit 16, as best seen in FIGURE 3, has two leg portions 17 'and 18 connected by a bight portion 19.

In the embodiment of the invention illustrated in FIG- URES 1 through 5, the U-shaped duct units are hollow box-like structures having flat parallel side walls 20 that are U shaped in plan, connected by transverse wall-s 21 joined to the marginal edges of the side walls.

As a result of the U-shaped formation of the duct unit 16, the legs thereof have adjacent inner edges and remote outer edges, and the leg portions 17 and 18 as well as their bight portion 19 connecting them are coplanar.

The cross sectional dimensions of the leg portions 17 and 18 are significant and important. As clearly shown in FIGURE 3, both leg portions have 'a uniform cross section throughout the length thereof, with the major dimension--i.e., the distance between the inner and outer edges of the leg-at least four times greater than the distance between its flat parallel side walls 20.

Each U- shaped duct unit has an inlet port 22 and an outlet port 23. These ports are located at the extremities of the outer remote edges of the leg units, and for structural strength the outer ends of the leg units are connected as by cross members 24.

The U-shaped duct units 16 are mounted in the flow passage in a manner which makes removal and replacement thereof a very simple matter. To this end, the wall 9 of the flow passage, which is adjacent to the edge of the U-shaped units defined by the outer ends of their legs, is removable. Upon removal of the wall 9 unrestricted access into the flow passage is afiorded. The several U-shaped duct units thus may be slid into the flow passage through the open rear thereof until their bight portions engage the partition wall 13 "and rest upon a supporting ledge 25 which projects rearwardly from the partition wall. In this position the inlet and outlet ports of the individual U-shaped duct units may be connected with inlet and outlet headers 26 and 27, respectively, these headers being rigidly secured in any suitable way to the structure defining the flow passage.

Preferably the inlet and outlet ports are in the form of nipples or pipe stubs 28 and 29, which align with and are secured to inwardly directed branches 26' and 27' extending from the inlet and outlet headers.

The connect-ion of the duct units with the inlet and outlet headers may be by means of flanges, as shown, or by directly welding the nipples 28 and 29 to the branches 2 6 and 27.

The described relationship of the several U-sh-aped duct units and the manner in which they are connected to the inlet and outlet headers, enables edgewise removal and replacement of any one of the units, so that a defective unit can be quickly and easily removed and replaced without in :anywise disturbing the remainder of the structure other than removing the wall 9 to gain access into the rear of the fio-w passage.

It is to be observed that the leg portions and the bight portion of each U-shaped duct unit are coplanar, and that the several U-shaped duot units are disposed edgewise to the path of the flowing hot combustion or flue gases or other donor medium entering the inlet 11 and leaving through the outlet 12. This disposition of the U- shaped duct units in the embodiment of the invention shown in FIGURES 1 and 2, places the leg 17 above the .4 leg 18, but more significantly, the leg in which the outlet port 23 of the duct unit is located is in the upstream end portion of the path of the gases, while the other leg in which the port 22 of the duct unit is located is in the downstream end portion of said path. Accordingly, the two media flow counter-current to one another, and because of the novel formation of the U-shaped duct units, this is a true counter-current flow.

By virtue of the dimensions of the leg portions of the unit, the recipient fluid, i.e., water or boiler fluid entering the lower leg 18 from the inlet header 26 can flow upwardly toward the inner edge of that lower leg then continue upwardly through the bight portion 20* and directly across the upper leg portion 17 to the outer edge thereof at which the outlet port connects to the outlet header 27 In that embodiment of the invention illustrated in FIGURES 1 and 2, where the U-shaped duct units are arranged with one leg above the other, it is preferable that the leg portions of the units be divergent, as shown. This assures natural circulation of the fluid flowing therethrough and precludes entrapment of air bubbles, since all of its upper surfaces slope upwardly toward the outlet port 23.

The embodiment of the invention illustrated in FIG- URES 4 and 5 is very similar to that shown in FIGURES l and 2, differing therefrom essentially only in the orientation of the U-shaped duct units. In this case the duct units may be said to be disposed upright with their bight portions lowermost and their legs extending upwardly therefrom, in which position they are supported by the partition Wall 13. Also, in this case it is the top wall 6' which is removable, so that in removing the duct units they must be lifted upwardly from the flow passage.

The inlet 11 .and the outlet 12 in the embodiment of the invention shown in FIGURES 4 and 5, are in the front and rear walls, respectively, of the flow passage. Accordingly the donor medium flows from left to right (in FIGURE 4) which again is counter-current to the flow of the recipi ent medium flowing through the several U-shaped duct units from the inlet header 26 to the outlet header 27.

It is preferable in the FIGURE 4 and 5 embodiment of the invention to communicate the outer ends of the leg portions of the U-shaped duct units at the outermost extremities thereofwhich, in this orientation, is at the top. This communication is conveniently effected by means of a pipe 30 provided with a restricting orifice 31 somewhere along the length thereof. Such restricted communication of the extremities of the leg portions preeludes the entrapment of air bubbles which could interfere with proper circulation.

In those embodiments of this invention illustrated in FIGURES 1 through 5, inclusive, the intended heat transfer takes place between hot combustion of line gases and the water or other liquid medium flowing through the U-shaped duct unit. Because of the wide difference in surface conductance between gaseous and liquid media, the duct units are provided with extended surface in the form of pins 33 secured to and projecting perpendicularly from the side walls 20 of the leg portions of the duct units.

The extended surface elements 33, like those of the copending application Serial No. 639,544, now Patent No. 3,385,356 granted May 28, 1968 may be formed of metal of high and low thermal conductivity, with most of the elements bimetallic in cross section, as shown in FIG- URES 14 and 15. Where the elements are bimetallic, they preferably have a core of high conductivity metal, such as copper, and an outer sheath of low conductivity metal such as iron or steel.

By properly proportioning the high and low conductivity metal in each element, the conductivity of the individual elements can be tailored to the heat available thereat. In this manner, the conductivity of the elements can be substantially matched to the downward gradient of the temperature difference between the two media. Thus, for instance, the elements that are located at the upstream portion of the path of the hot combustion or flue gases flowing through the heat exchanger preferably would have a cross section such as that shown in FIGURE 14, in which only a thin outer shell of iron surrounds a relatively large diameter copper core; those elements which are located in the medial portion of the path of the flowing gases would contain a lesser proportion of copper, as shown in FIGURE 15, while those at the downstream end of the combustion gas flow could be formed entirely of iron, as shown in FIGURE 16. This results in a substantial saving in cost since it materially reduces the amount of high conductivity metal used for the extended surface.

To assure substantially uniform velocity of the combustion or flue gases through the heat exchanger and thereby gain maximum abstraction of heat from the gases, the extended surface elements are preferably arranged in groups which differ from one another in the spacing of their elements. Thus the elements of the group designated A, in which the spacing is greatest, are located at the upstream end of the flow path Where the temperature is highest, while the remaining elements which constitute group B and in which the elements are more closely spaced, occupy the remainder of the path.

While in FIGURES 1-5, inclusive, the extended surface elements are divided into only two groups, it will be apparent that a larger number of groups with a smaller stepwise reduction in the spacing of the elements comprising the successive groups, will achieve greater uniformity in velocity. Such a succession of differently spaced elements is illustrated in FIGURES 10, 11, 12 and 13, in which case, however, the extended surface elements 33 are fixed to and project from tubes 35, as in the Dalin Patent No. 2,719,354. Crosswise of the tubes the elements 3 3 are uniformly spaced, as shown in FIGURE 9; but lengthwise of the tubes the spacing of the elements is not uniform. Thus, for instance, in FIGURE 10, the spacing of the elements along the length of the tube is maximum, while in FIGURE 13 it is minimum; and in FIGURES l1 and 12, the spacing is less than that of FIGURE 10, but greater than that of FIGURE 13.

The tubes shown in FIGURES 9-13, inclusive, with their extended surface elements thereon, are employed in that embodiment of the invention illustrated in FIGURES 6, 7 and 8. Here the invention is incorporated into a complete steam boiler or hot water generator having a primary or main combustion chamber 40 and a convection chamber 41. These two chambers are separated from one another by a partition wall formed of closely adjacent parallel water tubes 42, connected at their lower ends to a header 43 and at their upper ends to a header 44. The header 43 is one of a group of transverse tubes 45 which forms the bottom of the main combustion chamber 40, and the header 44 is spaced down from the top or ceiling of the combustion chamber which is defined by tubes 46, so that an opening 47 exists at the top of the partition wall through which the products of combustion and hot combustion or flue gases leave the main or primary combustion chamber and enter the convection chamber 41.

The front wall of the main combustion chamber is provided with an opening 48 into which an oil burner nozzle (not shown) or other heat source projects its flame into the combustion chamber. A flue gas outlet 49 in the lower portion of the rear wall 50 of the structure permits the spent gases to be discharged into the stack in the customary manner.

In the convection chamber 41 there are two U-shaped duct units, each indicated generally by the numeral 51. Although composed of tubes, these U-shaped duct units are the functional equivalent of the box-like U-shaped duct units 16 employed in those embodiments of the invention illustrated in FIGURES 15, inclusive. Accordingly, it should be understood that where the term U- shaped duct unit is employed in the specification and in the claims, it encompasses the units 51 as well as the box-like duct units 16 shown in FIGURES 1-5; andfor that matteralso the tubular units shown in FIGURES 17 and 18, to be hereinafter described.

As best seen in FIGURE 8, the U-shaped duct units 51 comprise two superimposed groups of tubes 35 connected by and communicated with one another through a common header 52. By the connection of the two superimposed groups of tubes 35 with their common header 52, they are physically held in coplanar relationship with one another and with the header 52. The two superimposed groups of tubes thus constitute the leg portion of the U- shaped duct, while the header 52 provides its bight portion.

The outer ends of each group of tubes are connected to and communicated with one another by separate headers 53 and 54, the former providing the inlet for the U-shaped duct unit and the latter providing its outlet. The header 53 is removably connected to a branch 55 rising from a transverse supply duct 56, as by means of pipe flanges, and the header 54 is connected with a transverse receiving duct 57. Because of the higher temperature obtaining at the connection of the header 54 with the receiving duct, it is preferable to avoid using a pipe flange connection.

As shown in FIGURE 8, the two groups of tubes 35 diverge from one another, so that fluid entering the same from the supply duct 56 can rise by natural circulation to the receiving duct 57, and it will be seen that the upward slope of the upper group of tubes 35 is substantially parallel with the header 44 which defines the bottom edge of the opening connecting the main or primary combustion chamber with the convection chamber.

To enable the U-shaped duct units 51 to be removed and replaced, that portion 60 of the side wall of the furnace which is adjacent to the separate headers 53 and 54 of the U-shaped duct units is removable. Upon removal of this wall portion 60 unrestricted access to the entire convection chamber is available so that by disconnecting the pipe flanges by which the header 53 is joined to its adjacent branch 55, and flame cutting through the upper portion of the header 54, either one of the two U-shaped duct units-which again may be considered modulescan be withdrawn from the convection chamber without in anywise interfering with or being obstructed by any other portion of the structure.

Preferably the removable wall portion has an inspection opening 61 therein provided with a suitable cover 62, and it is also preferable to provide the top wall or roof of the furnace unit with a removable trap door 63 above the convection chamber to enable access to the convection surfaces from above, for inspection and cleaning.

The countercurrent flow achieved in the embodiment of the invention illustrated in FIGURES 1-5, inclusive, is of course also present in the structure of FIGURES 6, 7 and 8, for-as shown in FIGURE 6the combustion gases flowing through the convection chamber 41 to the outlet 49 travel downwardly first across the upper group of tubes 35 and then across the lower group, and the water circulating through the tubes 35 enters at the bottom and leaves at the top.

Although fairly satisfactory counter-current flow is achieved even if the rate of flow through all of the tubes is the same, greater counter-current etficiency is assured by restricting the flow through those of the inlet tubes, i.e., lower group of tubes, that are swept by the lowest temperature gases. For this purpose the entrance into these tubes may be provided with flow restrictors 35', as shown in FIGURE 8.

As hereinbefore noted, the tubes 35 are equipped with extended surface elements 33; and-as shown in FIG- URES 1()l3, inclusive-the spacing of the elements is progressively less on different tubes. The tubes with the most widely spaced elements (FIGURE 10) are in the upstream end portion of the flow passage; those with the most closely spaced elements (FIGURE 13) are in the downstream end portion of the flow passage; and those tubes which occupy the intermediate portion of the flow passage have their elements spaced as shown in FIGURES 11 and 12. Hence the flowing gaseous medium encounters progressively increasing resistance so that despite its reduction in volume as heat is abstracted therefrom, the velocity of the flow is substantially constant along the entire length of the flow passage. This assures maximum abstraction of heat from the donor medium.

As hereinbefore described, the individual elements 33 may be bimetallic in cross section and composed of high and low conductivity metal, with the greatest proportion of high conductivity metal being used in the elements that are located where the temperatures are highest and vice versa.

The embodiment of the invention shown in FIGURES 17 and 18 is quite similar to that of FIGURES 6, 7 and 9, and differs therefrom only in thet substitution of a greater number of smaller diameter tubes 65 which have no extended surface thereon for the tubes 35; but the tubes 65 are arranged in staggered relationship, as shown in FIGURE 17, to attain more effective contact between the hot combustion gases flowing over the tubes and the water or boiler fluid flowing through the tubes.

As will appear from a comparison of FIGURES 6 and 17, the use of staggered smaller diameter tubes without extended surface thereon achieves a desirable compactness and, for some purposes, may be preferable to the arrangement employed in the embodiment of the invention shown in FIGURES 6, 7 and 8.

In the embodiments of the invention thus far described, the recipient fluid medium flows through the U-shaped duct units. This is the water-tube principle which is very well adapted to abstraction of heat from flue gases on the way to the stack. There are situations, though, for instance in the heating of oil, where it is preferable to have the donor medium flow through the tubes comprising the U-shaped duct units. FIGURES l9 and illustrate two forms of this fire tube embodiment of the invention. In each case, a flow passage 70 for the recipient fluid medium, for instance oil to be heated, is defined by top, bottom and side walls 71, 72 and 73, respectively. The flow through the passage 70 is from left to right.

The donor medium, for instance, hot flue gases or steam, flows through one or more tube-type U-shaped duct units '74 disposed within the flow passage. Each U-sha-ped duct units consists of a group of parallel inlet tubes 75 and a group of parallel outlet tubes 76, both connected at one end to a common header 77 and at the other end, resspectively to separate inlet and outlet headers 78 and 79. As in the other embodiments of the invention, the leg portions of the U-shaped duct units provided by the two groups of tubes and all of the headers are coplanar, and the units are disposed edgewise in the flow passage, with the inlet tubes 75 downstream and the outlet tubes 76 upstream with respect to the flow of the recipient medium along the flow passage, so that the desired countercurrent flow will be had.

To provide for ready removability of the U-shaped duct units, the top wall 71 of the flow passage has a hole 80 large enough to permit all of the uints to be removed at one time, but since it is desirable to have the individual units separately removable, the hole 80 is closed by a cover formed collectively by a plurality of edgewise engaging elongated wall sections 81, one for each U-shaped unit.

The individual wall sections 81 each have the two separate headers 78 and 79 of one unit mounted on the exterior thereof and the several headers 78 and 79 are detachably connected to inlet and outlet ducts 82 and 83, respectively, by which the hot donor medium is circulated to and from the tubes of the U-shaped duct units.

If hot combustion gases provide the donor fluid medium, the inside of the tubes inevitably become coated with foreign matter contained in the gases. This must be removed periodically, and to permit this to be done, the outer wall of the separate headers 78 and '79 has holes therein in line with the tubes, which holes are closed by removable cleanout plugs 74.

For the heating of oil, which has a relatively low coefiicient of surface conductance, extended surface elements are mounted on the exterior of the tubes. These elements preferably are of the bimetallic type and are ar ranged as previously described to achieve optimum heat transfer.

In the structure shown in FIGURE 19, the recipient fluid medium (oil) flows transversely first across the outlet tubes 76 and then across the inlet tubes 75 of the one or more U-shaped duct units in the flow passage, it being understood that these units are uniformly distributed across the width of the flow passage, i.e., the space between its side walls 73. This arrangement achieves the desired countercurrent flow of the two media in the way previously described.

A more effective countercurrent flow is accomplished with the structure shown in FIGURE 20. In this case, the flow of both media is lengthwise of the tubes, as a result of the provision of three bafiles 86, 87 and 88 for each U- shaped duct unit. The baffles 86 and 88 are respectively located upstream and downstream of the duct units, with respect to flow through the flow passage, and extend across the width of the flow passage from side wall to side wall thereof and from the bottom wall 72 which is adjacent to the common headers of the units, towards but short of the top wall 71. This provides an inlet 89 through which the recipient fluid medium reaches the U-shaped duct units, and an outlet 90 through which the heated recipient fluid medium leaves the heating zone.

The baffle 87 is located between the two groups of tubes of each U-shaped duct unit, and extends down from the top wall section 81 thereof towards but short of the units common header 77 to provide an opening 91 through which the recipient fluid medium flows from one group of tubes to the other. Since the U-shaped duct units are separately removable, it follows that the baffle 87 must be made up of separate edgwise engaging sections, one for each unit; but the upstream and downstream bafiies 86 and 88 can be and are of unitary construction.

From the foregoing description taken in connection with the accompanying drawings, it will be apparent to those skilled in this art that this invention provides an exceptionally compact heat exchanger especially well adapted for use as the convection surfaces of steam boilers and hot water generators and, in addition thereto, makes every portion of the water carrying duct system accessible for examination, and enables quick and easy removal and replacement of any duct unit that may be defective.

It should also be evident to those skilled in this art that the advantages of modular design are achieved by the U-shaped duct units of this invention, whether in the box-like form of FIGURES 1-5, inclusive, or the tubular form of FIGURES 6, 7, 8, 17 and 18; and to make maximum use of this modular approach, it is preferable for the duct unitsof each type-to be identical, regardless of their orientation in the flow passage. Thus, in the case of the box-like units of FIGURES 1-5, inclusive, even though the units are laterally arranged, as in FIGURES 1 and 2, and do not require the outer extremities of their leg portions to be communicated, the restricted communication which is necessary in the upright orientation of the units especially shown in FIGURE 4, should be included.

What is claimed as my invention is:

1. A heat exchanger for effecting indirect heat transfer between a donor fluid medium and a recipient fluid medium, comprising:

(A) wall means defining a flow passage for one of said media, with an inlet at one end of said passage and an outlet at the other end thereof, so that said one medium flows through said passage along a substantially defined path;

(B) a hollow box-like U-shaped duct unit having flat U-shaped side walls and transverse walls joined to the edges of the side walls, the transverse walls being of substantially uniform width so that the side walls are substantially parallel with one another and the legs and connecting bight portion of the U-shaped duct unit are coplanar, the legs of the U-shaped duct unit having adjacent inner edges and remote outer edges;

(C) means mounting the duct unit in said flow passage between its inlet and outlet and disposed edgewise to the path of the medium flowing through said passage, with the legs of the unit extending substantially transversely to said path, so that said medium first sweeps across one and then the other of the (D) means providing inlet and outlet ports for the U-shaped duct unit, one of which opens to one of its legs and the other to the other leg, so that one of the legs is an inlet leg and the other an outlet leg, the inlet leg being the one which is last swept by the fluid medium flowing through the flow passage, the distance between the inner and outer edges of each leg being several times as great as the distance between the side walls thereof, and the inlet and outlet ports being at the remote outer edge portions of the extremities of the legs, the box-like configuration of the duct unit enabling the fluid entering its inlet port to flow from the outer edge portion of the inlet leg across said leg to the inner edge portion thereof, through the connecting bight portion into the outlet leg and directly to the outer edge portion of the outlet leg for discharge through the outlet port, so that when the heat exchanger is used to abstract heat from the donor medium and the donor medium flows through the flow passage, the recipient fluid medium entering the inlet port may flow by natural circulation from any portion of the inlet leg to any portion of the outlet leg, whereby the flow of the donor and recipient media is truly countercurrent.

2. The structure of claim 1, wherein the inner and outer edges of the leg portions of said flat U-shaped side walls are substantially parallel and spaced apart a distance at least four times greater than the distance between said side walls.

3. The heat exchanger of claim 1, further characterized by extended surface elements fixed to the exterior of the side walls of the U-shaped duct unit and projecting into the path of the fluid medium flowing through the flow passage.

4. The structure of claim 3 wherein said extended surface elements are pins of uniform cross section projecting endwise from the side walls of the duct unit.

5. The structure of claim 4, wherein said extended surface elements are arranged in groups which differ from one another by the spacing between the elements, the group in which the elements are spaced farthest apart being at the upstream portion of the path of the fluid medium flowing through the flow passage and the group in which the elements are closest together being at the downstream end of said path, so that the velocity of said medium is substantially uniform along the entire length of said path thereof when the heat exchanger is used to abstract heat from said medium to thus assure maximum abstraction of heat therefrom.

References Cited UNITED STATES PATENTS 662,255 11/1900 Davies et al. 122-356 3,022,982 2/1962 Demalander -179 X 3,305,008 2/1967 Jacobs 165-179 X 2,222,496 11/ 1940 Belaiefl et al 16S145 X 2,938,712 5/1960 Pellmyr 165145 X 2,964,033 12/1960 Throcklmol'ton et al. l22-356 X ROBERT A. OLEARY, Primary Examiner. THEOPHIL W. STREULE, Assistant Examiner.

US. Cl. X.R. 

